Teenage boys have always been prone to mischief and bragging. Sometimes, the prank is a firecracker in a mailbox and the boasting occurs among friends.

But police and arson investigators say the Internet is changing the scale and scope of these pranks — and they are worried.

Recipes to create explosives out of common household items are easily accessible online, and teens can find a measure of celebrity through posting videos of their exploits on Web sites like YouTube.

Michael Chionchio, Delaware's assistant state fire marshal, has been tracking an uneven but discernable rise in incidents involving explosives. He said that since 2005, state fire investigators have investigated 68 incidents in which some type of improvised explosive device was used, resulting in the arrests of 13 adults and 32 juveniles.

"In years past, that resource — the Internet — was not available, and the construction of a device was more word-of-mouth or simulated from television shows or the movies," Chionchio said. "Just like every other Internet problem out there, it's hard to control. "

The most recent Delaware arrests, in January, involved two youths whom police say videotaped themselves blowing up a homemade bomb in a field, then posted it on YouTube.

They aren't alone. A recent search of YouTube turned up more than 92,600 videos of explosions, 1,910 of them involving homemade devices.

Teens are getting in trouble for Internet postings of other types of violent incidents, as well. Police arrested four Ossining, N.Y., teens in February on charges of first-degree gang assault after one of the youths used a cell-phone camera to photograph him and the others punching, kicking and slashing an 18-year-old in a church parking lot, then posted the graphic footage on YouTube.

"Teen boys have always done impulsive and sometimes stupid stunts," said Anastasia Goodstein, author of "Totally Wired: What Teens and Tweens Are Really Doing Online." "What's different is that this generation has access to inexpensive cameras and the Internet, giving them the ability to show off these stunts to the world and to get instant feedback and validation for what they're doing."

A YouTube spokeswoman said the company relies on users to police the site. The terms of use for YouTube state that bomb-making videos are prohibited, though such content still is posted and viewed every day.

"Our community flags content they feel is inappropriate, and once a video is flagged we review the material promptly and remove videos from the system if they violate our community guidelines," said Victoria Katsarou of YouTube.

The dangers of homemade explosives are obvious. A 19-year-old University of Miami student was killed in Florida in January when an 8-inch shard from a pipe-bomb explosion he was videotaping in his backyard glanced off a fence post and hit him in the face. In Elsmere, Del., last August, two teens, one 16 and the other 18, were arrested after a PVC pipe bomb was found near a children's swing set in a park.

Chionchio and others said it's not possible to prevent people from posting bomb recipes on the Internet without infringing on their free-speech rights. But Chionchio said more should be done to prevent people from displaying explosions on the Internet.

Monique Nelson, chief operating officer of Web Wise Kids, a nonprofit organization dedicated to educating adults and young people about how to safely use the Internet, said YouTube and other sites have not taken responsibility for allowing such videos to be posted.

"Most of the viewers are kids and they're not going to complain," Nelson said. "They, YouTube, have some responsibility in making sure that kind of stuff is not on there. There (also) has to be some cooperation from the parents."

Author Goodstein, of "Totally Wired," said the only real preventive measure is parental oversight: "Parents need to be more involved with what their kids are doing online, for them to be familiar with sites like YouTube, discuss what's appropriate and not appropriate to post and to insist on seeing their teen's channel."